22 FALL /WINTER 2022 △ continued from page 21 design processes following a combination of mechanistic (engineering properties and responses due to loads) and empirical (measured road performance) analyses. These led to the rise of mechanistic-empirical (ME) pavement design procedures. Eventually, AASHTO took up the effort to refine the existing design process and develop the AASHTO ME Design Guide used by several state departments of transportation today. While AASHTO was working on their guide, the National Asphalt Pavement Association contracted with Dr. David Newcomb at the University of Minnesota to develop a mechanistic design approach. With the assistance of Dr. David Timm, they developed the PerRoad program for perpetual pavement design. Since then, Dr. Timmhas refined the perpetual pavement design approach fromusing a single limiting strain value to the distribution of strains, based on his research at the National Center for Asphalt Technology (NCAT) Test Track in Auburn, Alabama. In less than a century, we have gone from an experience- based pavement design approach to highly sophisticated, computer-required, data-intensive design processes. Pavement DesignToday, Practically Speaking I have been in the pavement engineering field for nearly thirty years. I have been involved with the Transportation Research Board pavement committees. I have served on a number of other national and state pavement research committees. I was a part of the AASHTO task force that sunsetted the DARWin pavement design tool and wrote the initial PaveME application requirements. Yet, even I have been guilty of making pavement design more complicated than it needs to be. That changed in the 2000s, when Dr. Al-Qadi and I were both involved in a study that would later be called “Field Investigations of High Performance Pavements in Virginia” (VTRC 05-CR9), during which we observed and tested Virginia pavements for performance. By determining the common characteristics of well-performing pavement, we wanted to establish a premium pavement design for use on future VDOT projects. After evaluating eighteen sections (flexible, rigid and composite), we made a few key observations. First, long-term performance is tied to subgrade and sub-base. Most well-performing sections had a firm, stabilized subgrade and aggregate base layer. Two, these pavements were not experiencing issues with sub- surface drainage and loss of strength. Three, the flexible pavements’ thickness ranged from approximately eight to fifteen inches. Where pavement distress was present, it manifested from the top down or due to a mid-layer material (i.e., stripping) or construction (i.e., no bond) issues. Bottom-up fatigue was not believed to be a primary failure mechanism. While a follow-up project was recommended, it was never commissioned. However, over the next five years, I was involved in numerous pavement evaluations across Virginia to determine causes of failure and recommend pavement rehabilitation designs. The lessons learned from the 2005 project were verified and validated time and time again: in most cases, pavements too thin with inadequate subgrade/subbase support failed from the bottom up; and thicker pavements and those with a firm foundation failed from the top-down or mid-layer up. Nearly all these pavements were designed following VDOT’s procedure based on the AASHTO Road Test. I guess the Mesopotamians and Romans knew what they were doing when it came to well-performing pavements. Just a little too thin PAVEMENT DESIGN: A PHILOSOPHY CONSIDERED
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